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Originally appearing in Volume V19, Page 987 of the 1911 Encyclopedia Britannica.
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TERRIGENOUS DE-POSITS (formed in deep or shallow water close to land) 13 by a calcareous cement. Similar formations are found in the Mediterranean, where a dark mud predominates in the western part, passing into a grey, marly slime in the Tyrrhenian Basin and replaced by a typical calcareous ooze in the Eastern Basin. The bottom of the Black Sea is covered by a stiff blue mud in which Sir John Murray found much sulphide of iron,' grains or needles of pyrites making up nearly 5o% of the deposit, and there are also grains of amorphous calcium carbonate evidently precipitated from the water. The formation of the blue mud is largely aided by the putrefaction of organic matter, and as a result the water deeper than 120 fathoms is extra-ordinarily deficient in dissolved oxygen and abounds in sulphuretted hydrogen, the formation of which is brought about by a special bacterium, the only form of life found at depths greater than 120 fathoms in the Black Sea. In the Red Sea the " Pola " expedition discovered a calcareous ooze similar to that of the Mediterranean, and the formation of a stony crust by precipitation of calcium and magnesium carbonates may be recognized as giving origin to a recent dolomite. The terrigenous ingredients in the deposits become less and less abundant as one goes farther into the deep ocean and away from the continental margins. Still, according to Murray and Irvine, finely divided colloidal clay is to be found in all parts of the ocean however remote from land, though in very small amount, and there is less in tropical than in cooler waters. A minute fraction is always separating out of the water, and as a prodigious length of time may be accepted for the accomplishment of all the chemical and physical processes in the deep sea, we must take account of the gradual accumulation of even this infinitesimal precipitation. As well as the finest of terrigenous clay there is present in sea-water far from land a different clay derived from the decomposition of volcanic material. Volcanic dust thrown into the air settles out slowly, and some of the products of submarine and littoral volcanoes, like pumice-stone, possess a remarkable power of floating and may drift into any part of the ocean before they become waterlogged and sink. To this inconceivably slowly-growing deposit of inorganic material over the ocean floor there is added an overwhelmingly more rapid contribution of the remains of calcareous and siliceous planktonic and benthonic organisms, which tend to bury the slower accumulating material under a blanket of globigerina, pteropod, diatom or radiolarian ooze. When those deposits of organic origin are wanting or have been removed, the red clay composed of the mineral constituents is found alone. It is a remarkable geographical fact that on the rises and in the basins of moderate depth of the open ocean the organic oozes preponderate, but in the abysmal depressions below 2500 or 3000 fathoms, whether these lie in the middle or near the edges of the great ocean spaces, there is found only the red clay, with a minimum of calcium carbonate, though sometimes with a considerable admixture of the siliceous remains of radiolarians. Thus red clay and radiolarian ooze are distinguished as abyssal deposits in contradistinction to the epilophic calcareous oozes. Globigerina ooze was recognized as an important deposit as soon as the first successful deep-sea soundings had been made in the Atlantic. It was described simultaneously in 1853 by Bailey of West Point and Ehrenberg in Berlin. Murray and Renard define globigerina ooze as containing at least 30% of calcium carbonate, in which the remains of pelagic (not benthonic) foraminifera predominate and in which remains of pelagic mollusca such as pteropods and heteropods, ostracodes and also coccoliths (minute calcareous algae) may also occur. Not more than 25% of the deposit may consist of bottom-dwelling foraminifera, echini or worm-tubes, and as a rule these make up only from 9 to ro%. These peculiarities, combined with the striking absence of mineral constituents, distinguish the eupelagic globigerina ooze from the hemipelagic calcareous mud. Out of 118 samples of globigerina ooze obtained by the " Challenger " expedition 84 came from depths of 1500 to 25oo fathoms, 13 from depths of r000 to 1500 and only 16 from Scot. Geog. Mag., vol. 16 (190o), p. 695.depths greater than 2500 fathoms. Viewed as a whole this deposit may be taken as a partial precipitation of the plankton living in the upper waters of the open sea. A small proportion of organic matter including the fat globules of the plankton is mixed with the calcium carbonate, the amount according to Gumbel's analysis being about 1 part in r000. Secondary products, such as glauconite, phosphatic concretions and manganese nodules, occur though less frequently than in the hemipelagic sediments. Globigerina ooze is the characteristic deposit of the Atlantic Ocean, where it covers not less than 44,000,000 sq. km. (17,000,000 sq. statute m.). In the Indian Ocean the area covered is 31,000,000 sq. km. (12,000,000 sq. m.) and in the huge Pacific Ocean only 30,000,000 sq. km. (11,500,000 sq. m.). Pteropod ooze is merely a local variety of globigerina ooze in which the comparatively large but very delicate spindle-shaped shells of pteropods happen to abound. These shells do not retain their individuality at depths greater than 1400 or 1500 fathoms, and in fact pteropod ooze is only found in small patches on the ridges near the Azores, Antilles, Canaries, Sokotra, Nicobar, Fiji and the Paumotu islands, and on the central rise of the South Atlantic between Ascension and Tristan d'Acunha. Diatom ooze was recognized by Sir John Murray as the characteristic deposit in high latitudes in the Indian Ocean, and later it was found to be characteristic also of the corresponding parts of the Indian and Pacific covering a total area of about 22,000,000 sq. km. (8,5oo,000 sq. m.). It has been found sporadically near the Aleutian Islands, between the Philippines and Marianne Islands and to the south of the Galapagos group. It is made up to a large extent of the siliceous frustules of diatoms. It is usually yellowish-grey and often straw-coloured when wet, though when dried it becomes white and mealy. Red clay was discovered and named by Sir Wyville Thomson on the " Challenger " in 1873 when sounding in depths of 2700 fathoms on the way from the Canary Islands to St Thomas. The reddish colour comes from the presence of oxides of iron, and particles of manganese also occur in it, especially in the Pacific region, where the colour is more that of chocolate; but when it is mixed with globigerina ooze it is grey. Red clay is the deposit peculiar to the abysmal area; 70 carefully investigated samples collected by the " Challenger " came from an average depth of 2730 fathoms, 97 specimens collected by the " Tuscarora " came from an average depth of 286o fathoms, and 26 samples obtained by the " Albatross " in the Central Pacific came from an average depth of 2620 fathoms. Red clay has not yet been found in depths less than 2200 fathoms. The main ingredient of the deposit is a stiff clay which is plastic when fresh, but dries to a stony hardness. Isolated gritty fragments of minerals may be felt in the generally fine-grained homogeneous mass. The dredge often brings up large numbers of nodules formed upon sharks' teeth, the ear-bones of whales or turtles or small fragments of pumice or other volcanic ejecta, and all more or less incrusted with manganese oxide until the nodules vary in size from that of a potato to that of a man's head. A very interesting feature is the small proportion of calcium carbonate, the amount present being usually less as the depth is greater; red clay from depths exceeding 3000 fathoms does not contain so much as 1% of calcareous matter. Murray and Renard recognize the progressive diminution of carbonate of lime with increase of depth as a characteristic of all eupelagic deposits. The whole collection of 231 specimens of deep-sea deposits brought back by the " Challenger " shows the following general relationship: Proportion of Calcium Carbonate in Deep-Sea Deposits. 68 samples from less than 2000 fathoms = 6o-8o % 68 „ 2000-2500 65 2500-3000 more than 3000 In deep water there is a regular process of solution of the calcareous shells falling from the surface. Murray and Renard ascribe this to the greater abundance of carbonic acid in the 8 46.7 % 17.4 % 0.9 % deeper water, which aided by the increased pressure adds to the solvent power of the water for carbonate of lime. It is, however, a curious question how, considering the increase of carbonic acid by the decomposition of organic bodies and possible submarine exhalations of volcanic origin, the water has not in some places become saturated and a precipitate of amorphous calcium carbonate formed in the deepest water. The whole subject still requires investigation. Amongst the foreign material found embedded in the red clay are globules of meteoric iron, which are sometimes very abundant. Derived products in the form of crystals of phillipsite are not uncommon, but the most abundant of all are the incrustations of manganese oxide, as to the origin of which Murray and Renard are not fully clear. The manganese nodules afford the most ample proof of the prodigious period of time which has elapsed since the formation of the red clay began; the sharks' teeth and whales' ear-bones which serve as nuclei belong in some cases to extinct species or even to forms derived from those familiar in the fossils from the seas of the Tertiary period. This fact, together with the extraordinarily rare occurrence of such remains and meteoric particles in globigerina ooze, although there is no reason to suppose that at any one time they are unequally distributed over the ocean floor, can only be explained on the assumption that the rate of formation of the epilophic deposits through the accumulation of pelagic shells falling from the surface is rapid enough to bury the slow-gathering material which remains uncovered on the spaces where the red clay is forming at an almost infinitely slower rate. Sir John Murray believes that no more than a few feet of red clay have accumulated in the deepest depressions since the close of the Tertiary period. The red clay is the characteristic deposit of the Pacific Ocean, where about 1ot,000,000 sq. km. (39,000,000 sq. m.) are covered with it, while only 15,000,000 sq. km. (5,800,000 sq. m.) of the Indian Ocean and 14,000,000 sq. km. (5,400,000 sq. m.) of the Atlantic are occupied by this deposit; it is indeed the dominant submarine deposit of the water-hemisphere just as globigerina ooze is the dominant submarine deposit of the land-hemisphere. Radiolarian ooze was recognized as a distinct deposit and named by Sir John Murray on the " Challenger” expedition, but it may be viewed as red clay with an exceptionally large proportion of siliceous organic remains, especially those of the radiolarians which form part of the pelagic plankton. It does not occur in the Atlantic Ocean at all, and in the Indian Ocean it is only known round Cocos and Christmas Islands; but it is abundant in the Pacific, where it covers a large area between 50 and 15° N., westward from the coast of Central America to 165° W., and it is also found in patches north of the Samoa Islands, in the Marianne Trench and west of the Galapagos Islands. The total areas occupied by the various deposits according to the latest measurements of Krummel are as follows: Area of Submarine Deposits. Deposit. Sq. km. Sq. st. m. %. I. Littoral deposits 33,000,000 12,700,000 9.1 II. Hemipelagic „ . . . 55,700,000 21,500,000 15.4 1. Globigerina ooze 105,600,000 40,800,000 (29.2) 2. Pteropod ooze 1,400,000 500,000 (0.4) 3. Diatom ooze 23,200,000 8,900,000 (6.4) 4. Red clay 130,300,000 50,300,000 (36.1) 5. Radiolarian ooze . 12,200,000 4,700,000 (3'4) Geologists are agreed that littoral and hemipelagic deposits similar to those now forming are to be found in all geological systems, but the existence in the rocks of eupelagic deposits and especially of the abysmal red clay, though viewed by some as probable, is totally denied by others. There is even some hesitation in accepting the continuity of the chalk with the globigerina ooze of the modern ocean. From the obvious rarity of true abysmal rocks in the continental area Sir John Murray deduces the permanence of the oceans, which he holds havealways remained upon those portions of the earth's crust which they occupy now, and both J. Dana and Louis Agassiz had already arrived at the same conclusion. This theory accords well with the enormous lapse of time required in the accumulation of the red clay. Salts of Sea-water.—Sea-water differs from fresh water by its salt and bitter taste and by its unsuitability for the purposes of washing and cooking. The process of natural evaporation in the salines or salt gardens of the margin of warm seas made the composition of sea-salt familiar at a very early time, and common salt, Epsom salts, gypsum and magnesium chloride were recognized amongst its constituents. The analyses c f modern chemists have now revealed the existence of 32 out of the 8o known elements as existing dissolved in sea-water, and it is scarcely too much to say that the remaining elements also exist in minute traces which the available methods of analysis as yet fail to disclose. Many of the elements such as copper, lead, zinc, nickel, cobalt and manganese have only been found in the substance of sea-weeds and corals. Silver and gold also exist in solution in sea-water. Malaguti and Durocher estimate the silver in sea-water as 1 part in roo,000,000 or grain in 1430 gallons. If this estimate is correct there exists dissolved in the ocean a quantity of silver equal to 13,300 million metric tons, that is to say 46,700 times as much silver as has been produced from all the mines in the world from the discovery of America down to 1902. No quantitative determination of the amount of gold in solution is available. E. Sonnstadt detected gold by means of a colour test and roughly estimated the amount as r grain per ton of sea-water, and on this estimate all the projects for extracting gold from sea-water have been based. The elements in addition to oxygen which exist in largest amount in sea salt are chlorine, bromine, sulphur, potassium, sodium, calcium and magnesium. Since the earliest quantitative analyses of sea-water were made by Lavoisier in 1772, Bergman in 1774, Vogel in 1813 and Marcet in 1819 the view has been held that the salts are present in sea-water in the form in which they are deposited when the water is evaporated. The most numerous analyses have been carried out by Forchhammer, who dealt with 15o samples, and Dittmar, who made complete analyses of 77 samples obtained on the " Challenger " expedition. Dittmar showed that the average proportion of the salts in ocean water of 35 parts salts per thousand was as follows (calculated as parts per thousand of the sea-water, as percentage of the total salts and per hundred molecules of magnesium bromide) : The Salts in Ocean Water. Per 1000 Per cent. Per ul Molecules . Total Salts. MgBrz. Pacts Water. Common salt, sodium chloride (NaCI) . 27.213 77' 758 112,793 Magnesium chloride 3.807 1o•878 9,690 (MgC12) '. . . Magnesium sulphate 1.658 '4'737 3,338 (MgSO4) . . . . Gypsum, calcium sul- 1.26o 3.600 2,239 phate (CaSO4) . . Potassium sulphate o•863 2.465 1,200 (K2SO4) Calcium carbonate 0-123 0'345 298 (CaCO2) and residue Magnesium bromide 0.076 0.217 loo (MgBr2) . . . . 35.000 100.000 As Marcet had foreshadowed from the analysis of 14 samples in 1819, the larger series of exact analyses proved that the variations in the proportion of individual salts to the total salts are very small, and all analyses since Dittmar's have confirmed this result. Although the salts have been grouped in the above Comptes rendus, Acad. Sciences (Paris, 1859), 49, 463, 536. 2 Chemical News (187o), vol. 22, pp. 25, 44; (1872) vol. 26, P. 159• table it is not to be supposed that a dilute solution like sea-water contains all the ingredients thus arbitrarily combined. There must be considerable dissociation of molecules, and as a first approximation it may be taken that of ro molecules of most of the components about 9 (or in the case of magnesium sulphate 5) have been separated into their ions, and that it is only during slow concentration as in a natural saline that the ions combine to produce the various salts in the proportions set out in the above table. One can look on sea-water as a mixture of very dilute solutions of particular salts, each one of which after the lapse of sufficient time fills the whole space as if the other constituents did not exist, and this interdiffusion accounts easily for the uniformity of composition in the sea-water throughout the whole ocean, the only appreciable difference from point to point being the salinity or degree of concentration of the mixed solutions. The origin of the salt of the sea is attributed by some modern authorities entirely to the washing out of salts from the land by rain and rivers and the gradual concentration by evaporation in the oceans, and some (e.g. J. Jo-1y) go so far as to base a calculation of the age of the earth on the assumption that the ocean was originally filled with fresh water. This hypothesis, however, does not accord with the theory of the development of the earth from the state of a sphere of molten rock surrounded by an atmosphere of gaseous metals by which the first-formed clouds of aqueous vapour must have been absorbed. The great similarity between the salts of the ocean and the gaseous products of volcanic eruptions at the present time, rich in chlorides and sulphates of all kinds, is a strong argument for the ocean having been salt from the beginning. Two other facts are totally opposed to the origin of all the salinity of the oceans from the concentration of the washings of the land. The proportions of the salts of river and sea-water are quite different, as Julius Roth shows thus: Carbonates. Sulphates. Chlorides. River water . . . 8o 13 7 Sea water . . . . 0.2 10 89 The salts of salt lakes which have been formed in the areas of internal drainage in the hearts of the continents by the evaporation of river water are entirely different in composition from those of the sea, as the existence of the numerous natron and bitter lakes shows. Magnesium sulphate amounts to 4.7 % of the total salts of sea-water according to Dittmar, but to 23.6% of the salts of the Caspian according to Lebedinzeff; in the ocean magnesium chloride amounts to 10'9% of the total salts, in the Caspian only to 4.5%; on the other hand calcium sulphate in the ocean amounts to 3.6%, in the Caspian to 6.9%. This disparity makes it extremely difficult to view ocean water as merely a watery extract of the salts existing in the rocks of the land. The determination of salinity was formerly carried out by evaporating a weighed quantity of sea-water to dryness and weighing the residue. Forchhammer, however, pointed out that this method gave inexact and variable results, as in the act of evaporating to dryness hydrochloric acid is given off as the temperature is raised to expel the last of the water, and Tornoe found that carbonic acid was also liberated and that the loss of both acids was very variable. Tornoe vainly attempted to apply a correction for this loss by calculation; and subsequently S. P. L. Sorensen and Martin Knudsen after a careful investigation decided to abandon the old definition of salinity as the sum of all the dissolved solids in sea-water and to substitute for it the weight of the dissolved solids in moo parts by weight of sea-water on the assumption that all the bromine is replaced by its equivalent of chlorine, all the carbonate converted into oxide and the organic matter burnt. The advantage of the new definition lies in the fact that the estimation of the chlorine (or rather of the total halogen expressed as chlorine) is sufficientto determine the salinity by a very simple operation. According to Knudsen the salinity is given in weight per thousand parts by the expression S=o•o3o-}-r•8o5o Cl where S is the salinity and Cl the amount of total halogen in a sample. Such a simple formula is only possible because the salts of sea-water are of such uniform composition throughout the whole ocean that the chlorine bears a constant ratio to the total salinity as newly defined whatever the degree of concentration. This definition was adopted by the International Council for the Study of the Sea in 1902, and it has since been very widely accepted. Besides the determination of salinity by titration of the chlorides, the method of determination by the specific gravity of the sea-water is still often used. In the laboratory the specific gravity is determined in a pyknometer by actual weighing, and on board ship by the use of an areometer or hydrometer. Three types of areometer are in use: (1) the ordinary hydrometer of invariable weight with a direct reading scale, a set of from five to ten being necessary to cover the range of specific gravity from Iwo to 1.031 so as to take account of sea-water of all possible salinities; (2) the " Challenger " type of areometer designed by J. Y. Buchanan, which has an arbitrary scale and can be varied in weight by placing small metal rings on the stem so as to depress the scale to any desired depth in sea-water of any salinity, the specific gravity being calculated for each reading by dividing the total weight by the immersed volume; (3) the total immersion areometer, which has no scale and the weight of which can be adjusted so that the instrument can be brought so exactly to the specific gravity of the water sample that it remains immersed, neither floating nor sinking; this has the advantage of eliminating the effects of surface tension and in Fridtjof Nansen's pattern is capable of great precision. In all areometer work it is necessary to ascertain the temperature of the water sample under examination with great exactness, as the volume of the areometer as well as the specific gravity of the water varies with temperature. All determinations must accordingly be reduced to a standard temperature for comparison. Following the practice of J. Y. Buchanan on the "Challenger " it haq been usual for British investigators to calculate specific gravities for sea-water at 6o° F. compared with pure water at the maximum density point (39.2°) as unity. On the continent of Europe it has been more usual to take both at 17.5° C. (63.5° F.), which is expressed as " S a7:-", but for pyknometer work in all countries where the sample is cooled to 32° F. before weighing and pure water 'at 39.2° taken as unity the expression is (0°/4°). On the authority of the first meeting of the Inter-national Conference for the Study of the Northern European Seas at Stockholm in 1899 Martin Knudsen, assisted by Karl Forch and S. P. L. Sorensen, carried out a careful investigation of the relation between the amount of chlorine, the total salinity and the specific gravity of sea-water of different strengths including an entirely new determination of the thermal expansion of sea-water. The results are published in his Hydrographical Tables in a convenient form for use. The relations between the various conditions are set forth in the following equations where oe signifies the specific gravity of the sea-water in question at o° C., the standard at 4° being taken as 1000, S the salinity and Cl the chlorine, both expressed in parts by weight per mille. (I) Qo=–0.093+0'8149 S–0.000482 S2+o•0000068 S, (2) 7o=–0.069+1.4708 Cl–0.00157 C12+o•0000398 Cls (3) S = 0.030+1.8050 Cl. The temperature of maximum density of sea-water of any specific gravity was found by Knudsen to be given with sufficient accuracy for all practical purposes by the formula 0 =3'95–0.266ap, where 0 is the temperature of maximum density in degrees centigrade. The temperature of maximum density is lower as the concentration of the sea-water is greater, as is shown in the following table: Maximum Density Point of Sea-Water of Different Salinities. Salinity per mile . . o ro 20 30 35 40 Temperature 8° C . 3.95° 1'86° — 0'31° — 2'47° — 3 52° — 4'54° Density Q8 , . . o'oo° 8'18° 16'07° 24'15° 28'22° 32'32 Further Physical Properties of Sea-water.—The laws of physical chemistry relating to complex dilute solutions apply to sea-water, and hence there is a definite relation between the osmotic pressure, freezing-point, vapour tension and boiling-point by which when one of these constants is given the others can be calculated. The most easily observed is the freezing-point, and according to the very careful determinations of H. T. Hansen the freezing-point 7° C. varies with the degree of concentration according to the formula r = —o• oo86 -0.00646330 o — o•o0o I055vo2. According to the investigations of Svante Arrhenius the osmotic pressure in atmospheres may be obtained by simply multiplying the temp rature of freezing (r) by the factor -12.08, and it varies with temperature (t) according to the law which holds good for gaseous pressure. P, = Po(, +0.003671) and can thus be reduced to its value at o° C. Sigurd Stenius has calculated tables of osmotic pressure for sea-water of different degrees of concentration. The relation of the elevation of the boiling-point (t°) to the osmotic pressure (P) is very simply derived from the formula t=o•o24o7Fo, while the reduction of vapour pressure proportional to the concentration can be very easily obtained from the elevation of the boiling-point, or it may be obtained directly from tables of vapour tension. Physical Properties of Sea-Water. Salinity per mille . 10 20 30 35 40 Freezing-point (C.) -0.53 -1.07 -1.63 -1.91 -2.20 Osmotic pressure Po 6.4 13.0 19.7 23• I 26.6 atmospheres . Elevation of boiling- 0.16 0.31 0.47 0.56 0.64 point (C.) . . . Reduction of vapour 4.2 8.5 13•o 15.2 17.6 pressure (mm.) . . The importance of the osmotic pressure of sea-water in biology will be easily understood from the fact that a frog placed in sea-water loses water by exosmosis and soon becomes 20% lighter than its original weight, while a true salt-Water fish suddenly transferred to fresh water gains water by endosmosis, swells up and quickly succumbs. The elevation of the boiling-point is of little practical importance, but the reduction of vapour pressure means that sea-water evaporates more slowly than fresh water, and the more slowly the higher the salinity. Unfortunately no observations of evaporation from the surface of the open sea have been made and very few comparisons of the evaporation of salt and fresh water are on record. The fact that sea-water does evaporate more slowly than fresh water has been proved by the observations of Mazelle at Triest and of Okado in Azino (Japan). Their experiments show that in similar conditions the evaporation of sea-water amounts to from 70 to 91 % of the evaporation of fresh water, a fact of some importance in geophysics on account of the vast expanses of ocean the evaporation from which determines the rainfall and to a large extent the heat-transference in the atmosphere. The optical properties of sea-water are of immediate importance in biology, as they affect the penetration of sunlight into the depths. The refraction of light passing through sea-water is dependent on the salinity to the extent that the index of refraction is greater as the salinity increases. From isolated observations of J. Soret and E. Sarasin and longer series of experiments by Tornoe and Krummel this relation is shown to be so close that the salinity of a sample can be ascertained by determining the index of refraction. According to Krummel the following relations hold good at 18° C. for the monochromatic light of the D line of the sodium spectrum in units of the fifth decimal place. Relation of Refractive Index and Salinity. For water of salinity (per 0 10 20 30 35 40 mille) . . Refractive index 1.33000+ 308 502 694 885 981 1077 units of 5th decimal place The refractometer constructed by C. Pulfrich (of the firm of Zeiss, in Jena) has been successfully used by G. Schott and E. von Drygalski for the measurement of salinity at sea, and was found to have the same degree of accuracy as an areometer with the great advantage of being quite unaffected by the motion of the ship in a sea-way. The transparency of sea-water has frequently been measured at sea by the simple expedient of sinking white-painted disks and noting the depth at which they become invisible as the measure of the transparency of the water. For the north European seas disks of about 18 or 20 in. in diameter are sufficient for this purpose, but in the tropics, where the transparency is much greater, disks 3 ft. in diameter at least must be used or the angle of vision for the reflected light is too small. In shallow seas the transparency is always reduced in rough weather. In the North Sea north of the Dogger Bank, for instance, the disk is visible in calm weather to a depth of from ro to 16 fathoms, but in rough weather only to 61 fathoms. Knipovitch occasion-ally observed great transparency in the cold waters of the Murman Sea, where he could see the disk in as much as 25 fathoms, and a similar phenomenon has often been reported from Icelandic waters. The greatest transparency hitherto reported is in the eastern basin of the Mediterranean, where J. Luksch found the disk visible as a rule to from 22 to 27 fathoms, and off the Syrian coast even to 33 fathoms. In the open Atlantic there are great differences in transparency; Krummel observed a 6 ft. disk to depths of 31 and 36 fathoms in the Sargasso Sea, but in the cold currents of the north and also in the equatorial current the depth of visibility was only from 11 to 161 fathoms. In the tropical parts of the Indian and the Pacific Oceans the depth of visibility increases again to from 20 to 27 fathoms. Some allowance should be made for the elevation of the sun at the time of observation. Mill has shown that in the North Sea off the Firth of Forth the average depth of visibility of a disk in the winter half-year was 41 fathoms and in the summer half-year 61 fathoms, and, although the greater frequency of rough weather in winter might tend to obscure the effect, individual observations made it plain that the angle of the sun was the main factor in increasing the depth to which the disk remained visible. There are some observations on the transparency of sea-water of an entirely different character. Such, for instance, were those of Spindler and Wrangell in the Black Sea by sinking an electric lamp, those of Paul Regnard by measuring the change of electric resistance in a selenium cell or the chemical action of the light on a mixture of chlorine and hydrogen, by which he found a very rapid diminution in the intensity of light even in the surface layers of water. Many experiments have also been made by the use of photographic plates in order to find the greatest depth to which light penetrates. Fol and Sarasin detected the last traces of sunlight in the western Mediterranean at a depth of 254 to 260 fathoms, and Luksch in the eastern Mediterranean at 328 fathoms and in the Red Sea at 273 fathoms. The chief cause of the different depths to which light penetrates in sea-water is the varying turbidity due to the presence of mineral particles in suspension or to plankton. Schott gives the following as the result of measurements of transparency by means of a white disk at 23 stations in the open ocean, where quantitative observations of the plankton under r square metre of surface were made at the same time. Volume of Depth of Plankton. Visibility. Mean of 11 stations poor in plankton . 85 cc. 144 fathoms Mean of 12 stations rich in plankton . 53o „ „ J Any influence on transparency which may be exercised by the temperature or salinity of the water is quite in-significant. The colour of ocean water far from land is an almost pure blue, and all the variations of tint towards green are the result of local disturbances, the usual cause being turbidity of some kind, and this in the high seas is almost always due to swarms of plankton. The colour of sea-water as it is seen on board ship is most readily determined by comparison with the tints of Forel's xanthometer or colour scale, which consists of a series of glass tubes fixed like the rungs of a ladder in a frame and filled with a mixture of blue and yellow liquids in varying proportions. For this purpose the zero or pure blue is represented by a solution of 1 part of copper sulphate and 9 parts of ammonia in 190 parts of water. The yellow solution is made up of I part of neutral potassium chromate in 199 parts of water, and to give the various degrees of the scale, 1, 2, 3, 4, &c.,% of the yellow solution is mixed with 99, 98, 97, 96, &c.,% of the blue in successive tubes. Observations with the xanthometer have not hitherto been numerous, but it appears that the purest blue (o-r on Forel's scale) is found in the Sargasso Sea, in the North Atlantic and in similarly situated tropical or subtropical regions in the Indian and Pacific Oceans. The northern seas have an increasing tendency towards green, the Irminger Sea showing 5-9 Forel, while in the North Sea the water is usually a pure green (10-14 Forel), the western Mediterranean shows 5-9 Forel, but the eastern is as blue as the open ocean (0-2 Forel). A pure blue colour has been observed in the cold southern region, where the " Valdivia " found 0-2 Forel in 55° S. between ro° and 31° E., and even the water of the North Sea has been observed at times to be intensely blue. The blue of the sea-water as observed by the Forel scale has of course nothing to do with the blue appearance of any distant water surface due to the reflection of a cloudless sky. Over shallows even the water of the tropical oceans is always green. There is a distinct relationship between colour and transparency in the ocean; the most transparent water which is the most free from plankton is always the purest blue, while an increasing turbidity is usually associated with an increasing tint of green. The natural colour of pure sea-water is blue, and this is emphasized in deep and very clear water, which appears almost black to the eye. When a quantity of a fine white powder is thrown in, the light reflected by the white particles as they sink assumes an intense blue colour, and the experiments of J. Aitken with clear sea-water in long tubes leave no doubt on the subject. Discoloration of the water is often observed at sea, but that is always due to foreign substances. Brown or even blood-red stripes have been observed in the North Atlantic when swarms of the :opepod Calanus fanmarchicus were present; the brown alga Trichodesmium erythraeum, as its name suggests, can change the blue of the tropical seas to red; swarms of diatoms may produce olive-green patches in the ocean, while some other forms of minute life have at times been observed to give the colour of milk to large stretches of the ocean surface. On account of its salinity, sea-water has a smaller capacity for heat than pure water. According to Thoulet and Chevallier the specific heat diminishes as salinity increases, so that for io per mille salinity it is o•968, for 35 per mille it is only o•932, that of pure water being taken as unity. The thermal conductivity also diminishes as salinity increases, the conductivity for heat of sea-water of 35 per mille salinity being 4.2% less than that of pure water. This means that sea-water heats and cools somewhat more readily than pure water. The surface tension, on the other hand, is greater than that of pure water and increases with the salinity, according to Krtimmel, in the manner shown by the equation a= 77'09+0.0221 S at o° C., where a is the coefficient of surface tension and S the salinity in parts per thousand. The internal friction or viscosity of sea-water has also been shown by E. Ruppin to increase with the salinity. Thus at o° C. the viscosity of sea-water of 35 per mille salinity is 5.2% greater and at 25° C. 4% greater than that of pure water at the same temperatures; in absolute units the viscosity of sea-water at 25° C. is only half as great as it is at o° C. The compressibility of sea-water is not yet fully investigated.It varies not only to a marked degree with temperature, but also with the degree of pressure. Thus J. Y. Buchanan found a mean of zo experiments made by piezometers sunk in great depths on board the " Challenger " give a coefficient of compressibility K = 491 X Io-7; but six of these experiments made at depths of from 2740 to 3125 fathoms gave K=480X lo-7. The value usually adopted is K=450X lo -7. The compressibility is in itself very small, but so great in its effect on the density of deep water in situ that the specific gravity (0°°/4°) at 2000 fathoms increases by 0.017 and at 3000 fathoms by o•o26. In other words, water which has a specific gravity of I•o28o at the surface would at the same temperature have a specific gravity of I•o45o at 2000 and 1.0540 at 3000 fathoms. If the whole mass of water in the ocean were relieved from pressure its volume would expand from 319 million cub. m. to 321.7 million cub. m., which for a surface of 139.5 million sq. m. means an increased depth of too ft. The rate of propagation of sound depends on the compressibility, and in ocean water at the tropical temperature of 770 F. the speed is 1482.6 metres (486o ft.) per second, in Baltic water of 8 per mille salinity and a temperature of 5o° F. it is 1448.5 metres (4750 ft.) per second, that is to say, 41 times greater than the velocity of sound in air. This accounts for the great range of submarine sound signals, which can thus be very serviceable to navigation in foggy weather. The electrical conductivity of sea-water increases with the salinity; at 59° F. it is given according to E. Ruppin's formula as L=o•oo1465 S-0.00000978 S2+o•0000000876 S3 in reciprocal ohms. The radio-activity of sea-water is extraordinarily small; in-deed in samples taken from 50 fathoms in the Bay of Danzig it was imperceptible, and R. T. Strutt found that salt from evaporated sea-water did not contain one-third of the quantity of radium present in the water of the town supply in Cambridge. Dissolved Gases of Sea-water.—The water of the ocean, like any other liquid, absorbs a certain amount of the gases with which it is in contact, and thus sea-water contains dissolved oxygen, nitrogen and carbonic acid absorbed from the atmosphere. As Gay-Lussac and Humboldt showed in 18o5, gases are absorbed in less amount by a saline solution than by pure water. The first useful determinations of the dissolved gases of sea-water were made by Oskar Jacobsen in 1872. Since that time much work has been done, and the methods have been greatly improved. In the method now most generally practised, which was put forward by O. Pettersson in 1894, two portions of sea-water are collected in glass tubes which have been exhausted of air, coated internally with mercuric chloride to prevent the putrefaction of any organisms, and sealed up beforehand. The exhausted tube, when inserted in the water sample and the tip broken off, immediately fills, and is then sealed up so that the contents cannot change after collection. One portion is used for determining the oxygen and nitrogen, the other for the carbonic acid. The former determination is made by driving out the dissolved gases from solution and collecting them in a Torricellian vacuum, where the volume is measured after the carbonic acid has been removed. The oxygen is then absorbed by some appropriate means, and the volume of the nitrogen measured directly, that of the oxygen being given by difference. In the second portion the carbonic acid is driven out by means of a current of hydrogen, collected over mercury and absorbed by caustic potash. C. T. T. Fox, of the Central Laboratory of the International Council at Christiania, has investigated the relation of the atmospheric gases to sea-water by very exact experimental methods and arrived at the following expressions for the absorption of oxygen and nitrogen by sea-water of different degrees of concentration. The formulae show the number of cubic centimetres of gas absorbed by i litre of sea-water; t indicates the temperature in degrees centigrade and Cl the salinity as shown by the amount of chlorine per mille: O2 = I0.291-0.2809 1+0.006009 12-0.0000632 13- Cl(o.1161-0.003922 12+0.000063 t3) N2 =18.561-o•4282 1+0.0074527 t2–0.00005494 13 Cl(o•2149-0.007117 t2+0.0000931 13) In the case of ocean water with a salinity of 35 per mille, this gives for saturation with atmospheric gases in cc. per litre: at o° C. 15° C. 25° C. Oxygen . 8.o3 5'84 4.93 Nitrogen 14.40 II.12 9.78 The reduction of the absorption of gas by rise of temperature is thus seen to be considerable. As a rule the amount of both gases dissolved in sea-water is found to be that which is indicated by the temperature of the water in situ. Jacobsen on some occasions found water in the surface layers of the Baltic super-saturated with oxygen, which he ascribed to the action of the chlorophyll in vegetable plankton; in other cases when examining the nearly stagnant water from deep basins he found a deficiency of oxygen due no doubt to the withdrawal of oxygen from solution, by the respiration of the animals and by the oxidation of the deposits on the bottom. When these processes continue for a long time in deep water shut off from free circulation so that it does not become aerated by contact with the atmosphere the water becomes unfit to support the life of fishes, and when the accumulation of putrefying organic matter gives rise to sulphuretted hydrogen as in the Black Sea below 125 fathoms, life, other than bacterial, is impossible. The water from the greatest depths of the Black Sea, 116o fathoms, contains 6 cc. of sulphuretted hydrogen per litre. The distribution of dissolved oxygen in the depths of the open ocean is still very imperfectly known. Dittmar's analysis of the " Challenger " samples indicated an excess of oxygen in the surface water of high southern latitudes and a deficiency at depths below 50 fathoms. The facts regarding carbonic acid in sea-water are even less understood, for here we have to do not only with the solution of the gas but also with a chemical combination. On this account it is very difficult to know when all the gas is driven out of a sample of sea-water, and a much larger proportion is present than the partial pressure of the gas in the atmosphere and its co-efficient of absorption would indicate. These constants would lead one to expect to find o•5 cc. per litre at o° C. while as a matter of fact the amount absorbed approaches 50 ec. The form of combination is unstable and apparently variable, so that the quantities of free carbonic acid, bicarbonate and normal carbonate are liable to alter. Since 1851 it has been known that all sea-water has an alkaline reaction, and Tornoe defined the alkalinity of sea-water as the amount of carbonic acid which is necessary to convert the excess of bases into normal carbonate. The alkalinity of North Atlantic water of 35 per mille salinity is 26.86 cc. per litre, corresponding to a total amount of carbonic acid of 49•o7 cc. According to the researches of August Krogh,' the alkalinity is greatly increased by the ad-mixture of land water. This is proved by E. Ruppin's analysis of Baltic water, which has an alkalinity of r6 to r8 instead of the 5 or 6 which would be the amount proportional to the salinity, while the water of the Vistula and the Elbe with a salinity of o.1 per mille has an alkalinity of 28 or more. Thus the alkalinity serves as an index of the admixture of river water with sea-water. Carbonic acid passes from the atmosphere into the ocean as soon as its tension in the latter is the smaller; hence in this respect the ocean acts as a regulator. The amount of carbonic acid in solution may also be increased by submarine exhalations in regions of volcanic disturbance, but it must be remembered that the critical pressure for this gas is 73 atmospheres, which is reached at a depth of 400 fathoms, so that carbonic acid produced at the bottom of the ocean must be in liquid form. The respiration of marine animals in the depths of deep basins in which there is no circulation adds to the carbonic acid at the expense of the dissolved oxygen. This i5 frequently the case in fjord basins; for instance, in the Gullmar Fjord at a depth of 5o fathoms with water of 34.14 per mille salinity and Meddelelser om Granland (Copenhagen, 1904), p. 331.a temperature of 40.1° F., the carbonic acid amounts to 51'55 cc. per litre, and the oxygen only to 2.19 cc. Vegetable plankton in sunlight can reverse this process, assimilating the carbon of the carbonic acid and restoring the oxygen to solution, as was proved by Martin Knudsen and Ostenfeld in the case of diatoms. Little is known as yet of the distribution of carbonic acid in the oceans, but the amount present seems to increase with the salinity as shown by the four observations quoted: Water from Gulf of Finland of 3.2 per mille salinity =17.2 cc. CO2 at 4.1 ° C. Western Baltic of 14.2 =37.0 at 1•6° C. North Atlantic of 35'0 =49.0 „ at about Eastern Mediter- Io°C. raneanof 39'0 =53'0 at 26.7°C. Unfortunately the very numerous determinations of carbonic acid made by J. Y. Buchanan on the " Challenger " were vitiated by the incompleteness of the method employed, but they are none the less of value in showing clearly that the waters of the far south of the Indian Ocean are relatively rich in carbonic acid and the tropical areas deficient. Distribution of Salinity.—A great deal of material exists on which to base a study of the surface salinity of the oceans, and Schott's chart published in Petermanns Mitteilungen for 1902 incorporates the earlier work and substantially confirms the first trustworthy chart of the kind compiled by J. Y. Buchanan from the " Challenger " observations. In each of the three oceans there are two maxima of salinity—one in the north, the other in the south tropical belt, separated by a zone of minimum salinity in the equatorial region, and giving place poleward to regions of still lower salinity. The three oceans differ somewhat between themselves. The North Atlantic maximum is the highest with water of 37.9 per mille salinity; the maximum in the South Atlantic is 37.6; in the North Indian Ocean, 36.7; the South Indian Ocean, 36.4; the South Pacific, 36.9; and the North Pacific has the lowest maximum of all, only 35'9• The comparatively fresh equatorial belt of water, has a salinity of 35.0 to 34.5 in the Atlantic, 35.0 to 34'0 in the Indian Ocean, 34.5 in the Western and 33.5 in the Eastern Pacific. Taking each of the oceans as a whole the Atlantic has the highest general surface salinity with 35'37. The salinity of enclosed seas naturally varies much more than that of the open ocean. The saltest include the eastern Mediterranean with 39'5 per mille, the Red Sea with 41 to 43 per mille in the Gulf of Suez, and the Persian Gulf with 38. The fresher enclosed seas include the Malay and the East Asiatic fringing seas with 30 to 34.5 per mille, the Gulf of St Lawrence with 30 to 31, the North Sea with 35 north of the Dogger Bank diminishing to 32 further south, and the Baltic, which freshens rapidly from between 25 to 31 in the Skagerrak to 7 or 8 east-ward of Bornholm and to practically fresh water at the heads of the Gulfs of Bothnia and Finland. The Arctic Sea presents a great contrast between the salinity of the surface of the ice-free Norwegian Sea with 35 to 35.4 and that of the Central Polar Basin, which is dominated by river water and melted ice, and has a salinity less than 25 per mille in most parts. The average salinity of the whole surface of the oceans may be taken as 34.5 per mille. The vertical distribution of salinity has only recently been investigated systematically, as the earlier expeditions were not equipped with altogether trustworthy apparatus for collecting water samples at great depths. Two main types of water-bottle for collecting samples have been long in use. The older, devised by Hooke in 1667, is provided with valves above and below, both opening upward, through which the water passes freely during descent, but which are closed by some device on hauling up. The newer or slip water-bottle type consists of a cylinder allowed to drop on to a base-plate when a sample is to be collected. The first form of slip water-bottle due to Meyer retained the water merely by the weight of the cylinder pressing on the base-plate. J. Y. Buchanan introduced an improved form on the " Challenger,” also remaining closed by weight, the cylinder being very heavy and ground to fit the bevelled base-plate very accurately. H. R. Mill in 1885 devised a self-locking arrangement by which the bottle once closed was automatically locked and rendered watertight; H. L. Ekman made further improvements; and, finally, O. Pettersson and F. Nansen perfected the instrument, adapting it not only for enclosing a portion of water at any desired depth, but by a series of concentric divisions insulating in the central compartment water at the temperature it had at the moment of collection. By means of a weight dropped along the line the water-bottle can be shut and a sample enclosed at any desired depth. The use of a sliding weight is not recommended in depths much exceeding zoo fathoms on account of the time required and the risk of the line sagging at a low angle and so stopping the weight. In deep water the closing mechanism is usually actuated by a screw propeller which begins to work when the line is being hauled in and can be set so as to close the water-bottle in a very few fathoms. A small but heavy water-bottle has been devised by Martin Knudsen, provided with a pressure gauge or bathometer, by which samples may be collected from any moderate depth down to about 'co fathoms, on board a vessel going at full speed. This has made it possible to obtain many samples from moderate depths along a long line in a very short space of time. Sigsbee's small water-bottle on the double valve principle actuated by a propeller requires extremely skilful handling to enable it to give good results. As yet it is only possible to speak with confidence of the vertical distribution of salinity in the seas surrounding Europe, where there is a general increase of salinity with depth. For the open ocean the only quite trustworthy results are those obtained by the prince of Monaco in the North Atlantic, and by the recent Antarctic expeditions in the South Atlantic and South Indian Oceans. The observations made on the " Challenger " and " Gazelle," though enabling some perfectly sound general conclusions to be drawn, require to be supplemented. It appears, as J. Y. Buchanan pointed out in 1876, that the great contrasts in surface salinity between the tropical maxima and the equatorial minima give place at the moderate depth of 200 fathoms to a practically uniform salinity in all parts of the ocean. In the North Atlantic a strong submarine current flowing outward from the Mediterranean leaves the Strait of Gibraltar with a salinity of 38 per mille, and can be traced as far as Madeira and the Bay of Biscay in depths of from 600 to 2800 fathoms, still with a salinity of 35.6 per mille, whereas off the Azores at equal depths the salinity is from 0.5 to 0.7 per mille less. In the tropical and subtropical belts of the Atlantic and Indian Oceans south of the equator the salinity diminishes rapidly from the surface downwards, and at 500 fathoms reaches a minimum of 34.3 or 34.4 per mille; after that it increases again to 800 fathoms, where it is almost 34.7 or 34.8, and this salinity holds good to the bottom, even to the greatest depths, as was first shown by the " Gauss " and afterwards by the " Planet " between Durban and Ceylon. Our knowledge of the Pacific in this respect is still very imperfect, but it appears to be less salt than the other oceans at depths below 800 fathoms, as on the surface, the salinity at considerable depths being 34.6 to 34.7 in the western part of the ocean, and about 34.4 to 34.5 in the eastern, so that, although the data are by no means satisfactory, it is impossible to assign a mass-salinity of more than 34.7 per mille for the whole body of Pacific water. The causes of difference of salinity are mainly meteorological. The belt of equatorial minimum salinity corresponds with the excessively rainy belt of calms and of the equatorial counter-current, the salinity diminishing towards the east. The tropical maxima of salinity on the poleward side of the trade-winds coincide with the regions of minimum rainfall, high temperature, strong winds and consequently of maximum evaporation. Evaporation is naturally greatest in the enclosed seas of the nearly rainless subtropical zone such as the Mediterranean and Red Sea. Where the evaporation is at a minimum, the inflow of rivers from a large continental area and the precipitation from the atmosphere at a maximum, there is necessarily the greatest dilutionof the sea-water, the Baltic and the Arctic Sea being conspicuous examples. Temperature of the Oceans.—There is no difficulty in observing the temperature of the surface of the sea on board ship, the' only precautions required being to draw the water in a bucket which has not been heated in the sun in summer or exposed to frost in winter, to draw it well forward of any discharge pipes of the steamer, to place it in the shade on deck, insert the thermometer immediately and make the reading without delay. The measurement of temperature in the depths, unless a high-speed water-bottle be used, involves stopping the ship and employing thermometers of special construction. Many forms have been tried, but only three types are in general use. The first is the slow-action thermometer which was originally used with good effect by de Saussure in the Mediterranean in 1780. He covered the bulb of the thermometer with layers of non-conducting material and left it immersed at the desired depth for a very long time to enable it to take the temperature of its surroundings. When brought up again the thermometer retained its temperature so long that there was ample time to take a correct reading. Since 187o thermometers on this principle have been in use for regular observations at German coast and light-ship stations. Following the suggestion of Cavendish, Irving made observations of deep temperature on Phipps's Spitsbergen voyage of 1773 with a valved water-bottle, insulated by non-conducting material. A similar instrument gave excellent results in the hands of E. von Lenz on Kotzebue's second voyage of circumnavigation in 1823-1826. The last elaboration of the insulated slip water-bottle by Ekman, Nansen and Pettersson has produced an instrument of great perfection, in which the insulation is effected by layers of water between a series of concentric ebonite cylinders, all of which are closed both above and below when the apparatus encloses a sample, and each of which in turn must be warmed considerably before there is any rise of temperature in the chamber within. This can be used with certainty to •oz° C. for water down to 250 fathoms, after taking account of the slight disturbance produced by the expansion of the greatly compressed deep water. The second form of deep-sea thermometer is the self-registering maximum and - minimum on James Six's principle. These instruments must be constructed with the greatest care, but when well made in accordance with J. Y. Buchanan's large model they can be trusted to give a good account of the vertical distribution of temperature, provided the water grows cooler as the depth increases. They would act equally well if the water grew continually warmer as the depth increases, but they cannot give an exact account of a temperature inversion such as is produced when layers of warmer and colder water alternate. The third form is the outflow or reversing thermometer, first introduced by Aime, who used a very inconvenient form in the Mediterranean in 1841-1845, but greatly improved and simplified by Negretti and Zambra in 1875. The principle is to have a constriction in the tube above the bulb so proportioned that when the instrument is upright it acts in every way as an ordinary mercurial thermometer, but when it is inverted the thread of mercury breaks at the constriction, and the portion above the point runs down the now reversed tube and remains there as a measure of the temperature at the moment of turning over. For convenience in reading, the tube is graduated inverted, and when it is restored to its original position the mercury thread joins again and it acts as before. Various modifications of this form of thermometer have been made by Chabaud of Paris and others. It has the advantage over the thermometer on Six's principle that, being filled with mercury, it does not require such long immersion to take the temperature of the water. A correction has, of course, to be made for the expansion or con-traction of the mercury thread if the temperature of reading differs much from that of reversing. Magnaghi introduced a convenient method of inverting the thermometer by means of a propeller actuated on beginning to heave in the line, and this form is used for all work at great depths. For shallow water greater precision and certainty are obtained by using a lever actuated by a weight slipped down the line to cause the reversal, as in the patterns of Rung, Mill and others. All thermometers sunk into deep water must be protected against the enormous pressure to which they are exposed. This may be done by the method suggested by Arago in 1828, introduced by Aime in 1841 and again suggested by Glaisher in 1858, of sealing up the whole instrument in a glass tube exhausted of air; or, less effectively, by surrounding the bulb alone with a strong outer sheath of glass. In both forms it is usual to have the space between the bulb and the protecting sheath partly filled with mercury or alcohol to act as a conductor and reduce the time necessary for the thermometer to acquire the temperature of its surroundings. The warming of the ocean is due practically to solar radiation alone; such heat as may be received from the interior of the earth can only produce a small effect and is fairly uniformly distributed. On account of the high specific heat of sea-water the diurnal range of temperature at the surface is very small. According to A. Buchan's discussion of the two-hourly observations on the " Challenger " the total range between the daily maximum and minimum in the warmer seas is between 0•7° and o•8° F., and for the colder seas still less (0.2° F.), compared with 3.2° F. in the overlying air. The maximum usually occurs between 1 and 2.30 P.M., the minimum shortly before sunrise. The temperature of the surface water is generally a little higher than that of the overlying air, the daily average difference being about o•6° F., varying from 0.9° lower at 1 P.M. to 1.60 higher at 1 A.M. There are few observations available for ascertaining the depth to which warmth from the sun penetrates in the ocean. The investigations of Aline in 1845 and Hensen in 1889 indicate that the amount of cloud has a great effect. Aline showed that on a calm bright day in the Mediterranean the temperature rose o•1° C. between the early morning and noon at a depth of about 12 fathoms. Luksch deduced a much greater penetration of solar warmth from the comparison of observations at different hours at neighbouring stations in the eastern Mediterranean, but his methods were not exact enough to give confidence in the result. The penetration of warmth from the surface is effected by direct radiation, and by convection by particles rendered dense by evaporation increasing salinity. Conduction has practically no effect, for the coefficient of thermal conductivity in sea-water is so small that if a mass of sea-water were cooled to o° C. and the surface kept at a temperature of 3o° C., 6 months would elapse before a temperature of 15° C. was reached at the depth of 1.3 metres, 1 year at 1.85 metres, and to years at 5.8 metres. Great irregular variations in radiation and convection sometimes produce a remarkably abrupt change of temperature at a certain depth in calm water; the layer in which this sudden change occurs has been termed the Sprungschicht. How closely two bodies of water at different temperatures may come together is shown by the fact that in the Baltic in August between to and 11 fathoms there is sometimes a fall of temperature from 570 to 46.5° F. Such a condition of things is only possible in very calm weather, the action of waves having the effect of mixing the water to a considerable depth. After a storm the whole of the water in the North Sea assumes a homothermic condition, i.e. the temperature is the same from surface to bottom, and this occurs not only south of the Dogger Bank, where the condition is normal, but also, though less frequently, in the deeper water farther north. Similar effects are produced in narrow waters by the action of tidal currents, and the influence of a steady wind blowing on- or off-shore has a powerful effect in mixing the water. The warmest parts of the Indian Ocean and Western Pacific have a mean annual temperature of 82° to 84° F., but such high temperatures are not found in the tropical Atlantic. In the Indian Ocean between 15° N. and 5° S. the surface temperature in May averages 84° to 86° F., and in the Bay of Bengal the temperature is 86°, and no part of the Atlantic has so high a monthly mean temperature at any season. G. Schott's investigations how that the annual range of surface temperaturein the open ocean is greatest in 40° N., with 18.4° F., and in 3o° S., with 9.2° F.; on the contrary, near the equator it is less, only 40 F. in 1o° N., and in high latitudes it is also small, 5.2° F. in 5o° S. The figures quoted above are differences between the average surface temperatures of the warmest and of the coldest month. As to the absolute extremes of surface temperature, Sir John Murray points out that 9o° F. frequently occurs in the western part of the tropical Pacific, while among seas the Persian Gulf reaches 96° F., only 2° under blood-heat, and the Red Sea follows closely with a maximum of 940. The greatest change of temperature at any place has been recorded to the east of Neva Scotia, a minimum of 28° F. and a maximum of 8o°, and to the north-east of Japan with a minimum of 27° F. and a maximum of 83°. In those localities, however, it is not the same water which varies in temperature with the season, but the water of different warm and cold currents which periodic-ally occupy the same locality as they advance and retreat. The zones of surface temperature are arranged roughly parallel to the equator, especially in the southern hemisphere. Between 40° N. and 4o° S. the currents produce a considerable rearrangement of this simple order, the belts of warm water being wider on the western sides of the oceans and narrower on the eastern. The arrangement of the isotherms thus affords a basis for valuable deductions as to the direction of ocean currents. The surface temperature of the Atlantic is relatively lower than that of the other oceans when the whole area is considered. According to Krummel's calculation the proportional areas at a high temperature are as follows: Percentage of Ocean Surface with Temperature. Atlantic. Indian. Pacific. Over 770 F. (25° C) . . 22.4 38.o 40'1 Over 68° F. (20° C) . . . 50.1 51.7 58.4 This disparity results in some degree at least from the comparative narrowness of the inter-tropical Atlantic, and the absence of a cool northern area in the Indian Ocean. Krummel calculates that the mean temperature of the whole ocean surface is 63.3° F., while the mean sea-level temperature of the whole layer of air at the surface of the earth is given by Hann as 57.8° F. We are still ignorant of the depth to which the annual temperature wave penetrates in the open ocean, but observations in the Mediterranean enable us to form some opinion on the matter. The observations of Aline in 1845 and of Semmola in the Gulf of Naples in 1881 show that the surface water in winter cools until the whole mass of water from the surface to the bottom, in 1600 fathoms or more, assumes the same temperature. To-wards the end of summer the upper layers have been warmed to a depth which indicates how far the influence of solar radiation and convection have reached. Aime estimated this depth at 150-200 fathoms, while the observations of the Austrian expedition in the eastern Mediterranean found it to be from 200 to nearly 400 fathoms. In the Red Sea, where a similar seasonal change occurs, the depth to which the surface layer warms up is about 275 fathoms. The great difference in salinity between the surface and the deep water excludes the possibility of effective convection in the seas of northern Europe, and in the open ocean the currents which are felt everywhere, and especially those with a vertical component, must exercise a very disturbing influence on convection. The vertical distribution of temperature in the open ocean is much better known than that of salinity. The regional differences of temperatureat like depths become less as the depth increases. Thus at 300 fathoms greater differences than 90 F. hardly ever occur between 500 N. and 5o° S., in 800 fathoms the differences are less than 5.50 and in 1500 fathoms less than 20. Even in the tropics the high temperature of the surface is confined to a very shallow layer; thus in the Central Pacific where the surface temperature is 82° F. the temperature at loc. fathoms is only 5z° F. The whole ocean must thus form but a cold dwelling-place for the organisms of the deep sea. Sir John Murray calculates that at least 8o% of the water in the ocean has a temperature always less than 400 F., and a recent calculation by Krummel gave in fact a mean temperature of 390 F. for the whole ocean. The normal vertical distribution of temperature is illustrated in curve A of fig. 1, which represents a sounding in the South Atlantic; and this arrangement of a rapid fall of temperature giving place gradually to an extremely slow but steady diminution as depth increases is termed anathermic (av* , back, and Oep s, warm). Curve B shows the typical distribution of temperature in an enclosed sea, in this case the Sulu Basin of the Malay Sea, where from the level of the barrier to the bottom the temperature remains uniform or homothermic. Curve C shows a typical summer condition in the polar seas, where layers of sea-water at different temperatures are superimposed, the arrangement from the surface to 200 fathoms is termed £ I3 30 86 40 45 50 65 60 65 70 76r„ sw / -- zbn Sao 600 sbo 900 _ Iva voo _ 900 ,~„ 1 t 4000 - - ,o loco - 1 - _ too peso- i---, C ~cn our „ B 7Ra „ dichathermic (SL a, apart), from Iwo to z000 fathoms it is termed katathermic (Kara, down). In autumn the enclosed seas of high latitudes frequently present a thermal stratification in which a warm middle layer is sandwiched between a cold upper layer and a cold mass below, the arrangement being termed mesothermic (µEros, middle). The nature of the change of temperature with depth below 2500 fathoms is entirely dependent on the position of the sub-oceanic elevations, for the rises and ridges act as true submarine watersheds. As the Arctic Basin is shut off from the North Atlantic by ridges rising to within 300 fathoms of the surface and from the Pacific by the shallow shelf of the Bering Sea, and as the ice-laden East Greenland and Labrador currents consist of fresh surface water which cannot appreciably influence the underlying mass, the Arctic region has no practical effect upon the bottom temperature of the three great oceans, which is entirely dominated by the influence of the Antarctic. The existence of deep-lying and extensive rises or ridges in high southern latitudes has been indicated by the deep-sea temperature observations of Antarctic expeditions. Temperatures so low as 31.5° to 31.3° F, do not occur much beyond 5o° S. The "Belgica" even found a temperature of 33.10 F. in 61° S., 63° W., at a depth of zo18 fathoms. The conditions of temperature in the South Atlantic are characteristic. South of 550 S. in approximately 3000 fathoms the bottom temperature is 31.1° F.; in the Cape Trough it is 32.7° in 45° S., and 33.8° to 34.3° in 350 S., while north of the Walfisch Ridge and east of the South Atlantic Rise bottom temperatures of 36° to 36.7° F. prevail right northwards across the equator into the Bay of Biscay, showinga steady rise of bottom tempera-! ture as successive submarine elevations restrict communication with the Antarctic. On the other hand, in the more open Argentine Basin, which carries deep water far to the south, the bottom temperature in 4o° S. is only from 32.2° to 32.7° F., and the same low temperature continues throughout the Brazil Basin to the equator; but in the North American Basin from the West Indies to the Telegraph Plateau no satisfactory bottom temperature lower than 35.6° F. has been reported. On the floor of the Indian Ocean temperatures of 33.30 to 33.6° occur south of 350 S. in depths of 2700 fathoms or more, but north of 350 S. the prevailing bottom temperatures are from 34•o° to 34.3°• In similar depths in the Pacific south of the equator temperatures of 33.8° to 34.50 are found, and north of the equator bottom temperatures at the same depth increase to 3 5.1° in the neighbour-hood of the Aleutian Islands, again completely justifying the conclusion as to the Antarctic control of deep water temperature throughout the ocean. The marginal rises and continental shelves prevent this cold bottom water from penetrating into the depths of the enclosed and fringing seas. Thus in the Central American Sea below 930 fathoms, the depth on the bar, no water is found at a temperature lower than that prevailing in the open ocean at that depth, viz. 39.6° F., not even at the bottom of the great Bartlett Deep in 3439 fathoms. Such homothermic masses of water are characteristic of all deep enclosed seas. Thus in the Malay Sea the various minor seas or basins are homothermic below the depth of the rim, at the temperature prevailing at that depth in the open ocean: in the China Sea below 875 fathoms with 36.5° F.; in the Sulu Sea (depth 2550 fathoms) below 400 fathoms with 50.50 F.; in the Celebes Sea below 820 fathoms with 38.6° F.; in the Banda Sea below 902 fathoms with 37.90 F. In other enclosed seas which are shut off from the ocean by a very shallow sill the rule holds good that the homothermic water below the level of the sill is at the lowest temperature reached by the surface water in the coldest season of the year, provided always that the stratification of salinity is such as to permit of convection being set up. To this group belongs the Arctic Sea; the Norwegian Sea is homothermic below 55o fathoms at 29.8° F., but this cold water does not penetrate into the Arctic Basin on account of the ridge between Spitsbergen and Greenland, and there the water below 1400 fathoms has a temperature of 3o•6° to 30•7° F. because the surface layers of water are too light, on account of the low salinity due to ice-melting, to enable even the cold of a polar winter to set up a downward convection current. The Mediterranean Sea also belongs to this group; its various deep basins are homothermic (at the winter surface temperature) below the level of their respective sills—the Balearic Basin below 190 fathoms at 55° F.; the Eastern Basin below 270 fathoms at 55.9° F; the Ionian Sea at 56.3° F.; and at 56.7° south of Cyprus. Similarly in the Red Sea the water below 38o fathoms is homothermic at 70.7° F. An under-current flows out from the Red Sea through the Strait of Bab-el-Mandeb, and from the Mediterranean through the Strait of Gibraltar, raising the salinity as well as the temperature of the part of the ocean outside the gates of the respective seas. The action of the Red Sea water affects the whole of the Gulf of Aden and Arabian Sea, raising the temperature at the depth of 550 fathoms to 52° or 530 F. or 9 Fahrenheit degrees higher than the water of the Bay of Bengal at the same depth. The effect of the Mediterranean water in the North Atlantic does not require such large figures to express it, but is none the less extraordinarily far-reaching, as first indicated by the work of the " Challenger " and subsequently defined by H. N. Dickson's discussion of the observations of Wolfenden in the little sailing yacht " Silver Belle.” The temperature at 550 fathoms is raised to 490 or 50° F. between Madeira and the Biscay Shelf, i.e. 5.40 F. above the temperature at the same depth off the Azores. In shallow seas such as the North Sea and the British fringing seas, where tidal currents run strong, there is a general mixing together of the surface and deeper water, thus making the arrangement of vertical temperature anathermic in summer and katathermic in winter, while at the transitional periods in spring and autumn it is practically homothermic. Thus at Station E2 of the international series at the mouth of the English Channel in 490 27' N., 40 42' W., the following distribution of temperature F. has been observed by Matthews: August November February May 1904. 1 904. 1905. 1905. Surface . . 63.7° 56.2° 50.7° 51.3° 164 fathoms 55.5 56.5 50.8 50.5 52 fathoms . . 55.4 56.5 50.8 50.5 It is noticeable that there is a marked vertical temperature gradient only at the end of summer when a warm surface layer is formed, though in August 1904 that was only 8 fathoms thick. In small nearly land-locked basins shut off from one another by bars rising to within a short distance of the surface and affected both by strong tidal currents and by a considerable admixture of land water, the contrasts of vertical distribution of temperature with the seasons are strongly marked, and there are also great unperiodic changes effected mainly by wind, as is shown by the investigations of H. R. Mill in the Clyde Sea Area, and of O. Pettersson, J. Hjort and Helland-Hansen in the Scandinavian fjords. Sea Ice.—The freezing-point of sea-water is lower as the salinity increases and normal sea-water of 35 per mille salinity freezes at 28.6° F. Experience shows that sea-water can be cooled considerably below the freezing-point without freezing if there is no ice or snow in contact with it. Freezing takes place by the formation of pure ice in flat crystalline plates of the -hexagonal system, which form in perpendicular planes and unite in bundles to form grains so that a thick covering of ice exhibits a fibrous structure. It is only the water that freezes; the dissolved salts are excluded in the process in a regular order according to temperature. At temperatures about 17° F. sodium sulphate is the first ingredient of the salts to separate out, potassium chloride follows at 120 F., sodium chloride at — 7.4° F., magnesium chloride at — 28.5° F., and, as O. Pettersson was the first to point out, calcium chloride not until -67° F. During the rapid formation of ice the still unfrozen brine is often imprisoned between the little plates of frozen water; hence without some special treatment sea-ice is not suitable as a source of drinking water. After long continued frost the last of the included brine may be frozen and the salts driven out in crystals on the surface; these crystals are known to polar explorers by the Siberian name of rassol. Ice is a very poor conductor of heat and accordingly protects the surface of the water beneath from rapid cooling; hence new-f .,rmed pancake ice does not increase excessively in thickness in one winter, and even in the centre of the Arctic Basin the ice-covering only amounts to 6 or at most 9 ft. in the course of a year, while in the Antarctic regions the season's growth is only half as great; in the latter also the accumulated snow is an important factor in the thickness of the ice, and snow is an even worse conductor of heat. The influence of wind and tide breaks up the frozen surface of the sea, and sheets yielding to the pressures slide over or under one another and are worked together into a hummocky ice-pack, the irregularities on the surface of which, caused by repeated fractures and collisions, may be from to to 20 ft. high. Such formations, termed toross by the Russians, may extend under water, according to Makaroff's investigations, to at least an equal depth. Such old sea-ice when prevented from escaping forms the palaeocrystic sea of Nares; but, as a rule, it is carried southward in the East Greenland and Labrador currents, and melted in the warmer seas of lower latitudes. In the southern hemisphere the ice-pack forms a nearly continuous fence around the Antarctic continent. Pack-ice forms regularly in the inner part of the Baltic every winter, but not in the Norwegian fjords. Evenin the Mediterranean sea-ice is formed annually in the northern part of the Black Sea, and more rarely in the Gulf of Salonica and at the head of the Adriatic off Triest. Hudson Bay is blocked by ice for a great part of the year, and the Gulf of St Lawrence is blocked every winter. Ice also clothes the continental shores of the northern fringing seas of eastern Asia. In addition to sea-ice, icebergs which are of land origin occur at sea. In the north, icebergs break off, as a rule, from the ends of the great glaciers of Greenland, and in the far south from the edge of the great Antarctic ice-barrier.. The latter often gives birth to prodigious icebergs and ice islands, which are carried northward by ocean currents, nearly as far as the tropical zone before they melt. Thus in December 1906, an iceberg was seen off the mouth of the La Plata in 38° S., and in 1840 one was seen near Cape Agulhas in 350 S. The Antarctic icebergs are of tabular form and much larger than those of Greenland, but in either case an iceberg rising to 200 ft. above sea-level is uncommon, and one exceeding 300 ft. is very rare. The Greenland icebergs are carried by the Labrador current across the great banks of Newfoundland, where they are often very numerous in the months from February to August, when they constitute a danger to shipping as far south as 40° N. No icebergs occur in the North Pacific, and none has ever been reported nearer the coasts of Europe than off the Orkney Islands, and there only once, in 1836. Oceanic Circulation.—Although observations on marine currents were made near land or between islands even in antiquity, accurate observations on the high seas have only been possible since chronometers furnished a practicable method of determining longitude, i.e. from the time of Cook, the circumnavigator. The difference between the position as determined astronomically and by dead-reckoning gives an excellent idea of the general direction and velocity of the surface currents. The first comprehensive study of the currents of the Atlantic was that carried out by James Rennell (1790-1830), and since that time Findlay in his Directories, Heinrich Berghaus, Maury and the officials of the various Hydrographic Departments have produced increasingly accurate descriptions of the currents of the whole ocean, largely from material supplied by merchant captains. Direct observations of currents in the open sea are difficult, and even when the ship is anchored the veering and rolling of the vessel produce disturbances that greatly affect the result. Such current-meters as those used by Aime in 1841 and by Irminger since 1858 only gave the direction of the deeper current by comparison with the surface current at the time of observation. Later apparatus, such as Pettersson's bifilar current-meter or his more recent electric-photographic apparatus, and Nansen and Ekman's propeller current-meter, measure both the direction and the velocity at any moderate depth from an anchored vessel. One of the indirect methods of investigating currents is by taking account of the initial temperature of the current and following it by the thermometer throughout its course; hence the familiar contrast between warm and cold currents, of which the Gulf Stream and the Labrador current are types. Benjamin Franklin in 1775 and Charles Blagden in 1781, by means of numerous observations of temperature made on board the packets plying on the Atlantic passage, determined the boundaries of these two currents and their seasonal variations with considerable precision. The differences of salinity support this method, and, especially in the northern European seas, often prove a sharper criterion of the boundaries than temperature itself; this is especially the case at the entrance to the Baltic. Evidence drawn from drift-wood, wrecks or special drift bottles is less distinct but still interesting and often useful; this method. of investigation includes the use of icebergs as indicators of the trend of currents and also of plankton, the minute swimming or drifting organisms so abundant at the surface of the sea. The general lines of the currents of the oceans are fairly well understood, and along the most frequented ocean routes the larger seasonal variations have also been ascertained. The general scheme of ocean currents depends on the prevailing winds taken in conjunction with the configuration of the coast and its submarine approaches. The trade-wind regions correspond pretty closely with westward-flowing currents, while in the equatorial calm belts there are eastward-running counter-currents, these lying north of the equator in the Atlantic and Pacific, but south of the equator in the Indian Ocean. In the region of the westerly winds on the poleward side of 40° N. and S. the currents again flow generally eastward. A cyclonic circulation of the atmosphere is associated with a cyclonic circulation of the water of the ocean, as is well shown in the Norwegian Sea and North Atlantic between the Azores and Greenland. Where the trade-winds heap up the surface water against the east coasts of the continents the currents turn poleward. The north equatorial current divides into the current entering the Caribbean Sea and issuing thence by the Strait of Florida as the Gulf Stream, and the Antilles current passing to the north of the Antilles. Both currents unite off the coast of the United States and run northward, turning towards the east when they come within the influence of the prevailing westerly winds. In a similar manner the Brazil current, the Agulhas current and the East Australian current originate from the drift of the south-east trades, and in the North Pacific the Japan current arises from the north-east trade drift. The west-wind drifts on the poleward side carry back part of the water southward to reunite with the equatorial current, and thus there is set up an anticyclonic circulation of water between ro° and 40° in each hemisphere, the movement of the water corresponding very closely with that of the wind. The coincidence of wind and current direction is most marked in the region of alternating monsoons in the north of the Indian Ocean and in the Malay Sea. The accordance of wind and currents is so obvious that it was fully recognized by seafaring men in the time of the first circumnavigators. Modern investigations have shown, however, that the relationship is by no means so simple as appears at first. We must remember that the ocean is a continuous sheet of water of a certain depth, and the conditions of continuity which hold good for all fluids require that there should be no vacant space within it; hence if a single water particle is set in motion, the whole ocean must respond, as Varenius pointed out in r65o. Thus all the water carried forward by any current must have the place it left immediately occupied by water from another place, so that only a complete system of circulation can exist in the ocean. Further, all water particles when moving undergo a deviation from a straight path due to the forces set up by the rotation of the earth deflecting them towards the right as they move in the northern hemisphere and towards the left in the southern. This deflecting force is directly proportional to the velocity and the mass of the particle and also to the sine of the latitude; hence it is zero at the equator and comes to a maximum at the poles. When the wind acts on the surface of the sea it drives before it the particles of the surface layer of water, and, as these cannot be parted from those immediately beneath, the internal friction of the fluid causes the propelling impulse to act through a considerable depth, and if the wind continued long enough it would ultimately set the whole mass of the ocean in motion right down to the bottom. The current set up by the grip of the wind sweeping over the surface is deflected by the earth's rotation about 450 to the right of the direction of the wind in the northern hemisphere and to the left in the southern. The deeper layers lag behind the upper in deflection and the velocity of the current rapidly diminishes in consequence. The older theory of the origin of drift currents enunciated by Zoppritz in 1878 was modified as indicated above by Nansen in 1901, and Walfrid Ekman subsequently went further. He showed that at a certain depth the direction of the current becomes exactly the opposite of that which has been imposed by deflection on the surface current, and the strength is reduced thereby to only one-twentieth of that at the surface. He called the depth at which the opposed direction is attained the drift-current depth, and he found it to be dependent on the velocity of the surface current and on the latitude. According to Ekman's calculation with a trade-wind blowing at 16 m. per hour, thedrift-current depth in latitude 5° would be approximately 104 fathoms, in latitude 15°, 55 fathoms, and in latitude 45° only from 33 to 38 fathoms. A strong wind of 38 m. an hour would produce a drift-current depth of 82 fathoms in latitude 45°, and a light breeze of 3 m, an hour only 22 fathoms. It follows that a pure trade-wind drift cannot reach to any great depth, and this seems to be confirmed by observation, as when tow-nets are sunk to depths of 50 fathoms and more in the region of the equatorial current they always show a strong drift away from the side of the ship, the ship itself following the surface current. Ekman shows further that in a pure drift current the mean direction of the whole mass of the current is perpendicular to the direction of the wind which sets it in motion. This produces a heaping-up of warm water towards the middle of the anti-cyclonic current circulation between to° and 40°, and on the other hand an updraught of deep water along the outer side of the cyclonic currents. The latter phenomenon is most clearly shown by the stripes of cold water along the west coasts of Africa and America, the current running along the coast tending to draw its water away seawards on the surface and the principle of continuity requiring the updraught of the cool deep layers to take its place. For this reason the up-welling coastal water is coldest close to the shore, and hence it only appears on the Somali coast during the south-west monsoon. On the flat coasts of Europe the influence of on-shore wind in driving in warm water, and of off-shore wind in producing an updraught of cold water, has long been familiar to bathers. In a similar way updraughts of cold water to the surface occur in the neighbour-hood of the equator, especially in the Central Atlantic and Pacific. When a drift-current impinges directly upon a coast there is a heaping up of surface water, giving rise to a counter-current in the depths, which maintains the level, and this counter-current, although subject to deflection on account of the rotation of the earth, is deflected much less than a pure drift-current would be. Such currents, due to the banking up of water, have a large share in setting the depths of the sea in motion, and so securing the vertical circulation and ventilation of the ocean. The difference in density which occurs between one part of the ocean and another, shares with the wind in the production of currents. Vertical movements are also produced by difference of temperature in the water, but these can only be feeble, as below r000 fathoms the temperature differences between tropical and polar waters are very small. If we assign to a column of water at the equator the density S1=1'022 at the surface and 1•o28 at r000 fathoms, or an average of 1.025, and to a column of water at the polar circle a mean density of 1.028, there would result a difference of level equal to (1.028—1.025) X 1eoo= 3 fathoms in a distance from the equator to the polar circle of some 4600 m. A gradient like this, only r in 1,350,000, could give rise only to an extremely feeble surface current polewards and an extremely feeble deep current towards the equator. If there were strong currents at the bottom of the ocean the uniform accumulation of the deposit of minute shells of globigerina and radiolarian ooze would be impossible, the rises and ridges would necessarily be swept clear of them, and the fact that this is not the case shows that from whatever cause the waters of the depths are set in motion, that motion must be of the most deliberate and gentlest kind. In exceptional cases, when a strong deep current does flow over a rise, as in the case of the Wyville Thomson Ridge, the•bottom is swept clear of fine sediment. Strongly marked differences in density are produced by the melting of sea-ice, and this is of particular importance in the case of the great ice barrier round the Antarctic continent. O. Pettersson has made a careful study of ice melting as a motive power in oceanic circulation, and points out that it acts in two ways: on the surface it produces dilution of the water, forming a fresh layer and causing an outflow seaward of surface water with very low salinity; towards the deep water it produces a strong cooling effect, leading to increase of density and sinking of the chilled layers. Both actions result in the drawing in of an intermediate layer of water from a distance which takes part in the double system of vertical circulation as is indicated in fig. 2. The actual direction of this circulation is strongly modified by the influence of the earth's rotation. The existence of a layer of water of low salinity at a depth of 500 fathoms in the tropical oceans of the southern hemisphere is to be referred to this action of the melting ice of the Antarctic regions. Pettersson's view that ice-melting dominates the whole circulation of the oceans and regulates in particular the currents of the seas round northern Europe must, however, be looked on as carrying the explanation too far. Differences of density between the waters of enclosed seas and of the ocean are brought about in some instances by concentration of the water of the sea on account of active evaporation, and in other instances by dilution on account of the great influx of land water. A very powerful vertical circulation is thus set up between enclosed seas and the outer ocean. The very dense water of the Red Sea and the Mediterranean makes the column of water salter and heavier and the level lower than in the ocean beyond the straits. Hence a strong surface current sets inwards through the Straits of Bab-el-Mandeb and Gibraltar, while an undercurrent flows outwards, raising the temperature and salinity of the ocean for a long distance beyond the straits. 0:efh Ice -1~ 1° 2 3 1° 5° a 7'8° id 12 0''gh o o © ~ \foo too wo ~ ~s \ too too :'1 90• 40• 400 600 too ` eoo eoo , a too too eoo eoo W%f// A 900 90• eeo foo• tfo• ~~ ~~s ^06 ~_ oc 45 S.Lat. 4o 61 S.Lat. co 55` So Through the Bosporus and Dardanelles at the entrance of the Black Sea, and through the sound and belts at the entrance of the Baltic, streams of fresh surface-water flow outwards to the salter Mediterranean and North Sea, while salter water enters in each case as an undercurrent. Wind and tide greatly alter the strength of these currents due to difference of density, and the surface outflow may either be stopped or, in the case of the belts, actually reversed by a strong and steady wind. Both outflowing and inflowing currents are subject to the deflection towards the right imposed by the earth's rotation. Modern oceanography has found means to calculate quantitatively the circulatory movements produced by wind and the distribution of temperature and salinity not only at the surface but in deep water. The methods first suggested by H. Mohn and subsequently elaborated by V. Bjerknes have been usefully applied in many cases, but they cannot take the place of direct observations of currents and of the fundamental processes and conditions underlying them. The determination of the exact relationship of cause and effect in the origin of ocean currents is a matter of great practical importance. The researches of Pettersson, Meinardus, H. N. Dickson and others leave no doubt, for example, that the variations in the intensity of the Gulf Stream, whether these be measured by the change in the strength of the current or in the heat stored in the water, produce great variations in the character of the weather of northern Europe. The connexion between variations of current strength and the conditions of existence and distribution of plankton are no less important, especially as they act directly or indirectly on the life-conditions of food fishes. AutasoRarlES.—General: M. F. Maury, The Physical Geography of the Sea and its Meteorology (New York and London, 186o); J. J. Wild, Thalassa: an Essay on the Depths, Temperature and Currents of the Ocean (London, 1877) ; C. D. Sigsbee, Deep-sea Sounding and Dredging (Washington, 188o) ; O. Krummel, Handbuch der Ozeano-graphie (2 vols., Stuttgart, 1907) ; O. Krtimmel, Der Ozean (L4eipzig, 1902) ; J. Thoulet, Oceanographic (2 vols., Paris), vol. i. Statique (189o), vol. ii. Dynamique (1896) ; J. Thoulet, L'Ocean, ses lois et ses problemes (Paris, 1904) ; J. Thoulet, Guide de l'oceanogrephie pratique (Paris, 1895); J. Walther, Allgemeine Meereskunde (Leipzig, 1893); Luigi Hugues, Oceanograf a (Turin, 1904.) ; Sir J. Prestwich, " Tables of Temperatures of the Sea at Different Depths ... made between the years 1749 and 1868," Phil. Trans. clxv. (1876), 639-670; A. Buchan, " Specific Gravities and Oceanic Circulation," Trans. Roy. Soc. Edinburgh, xxxiv. (1896), 317-342; Sir John Murray, " Presidential Address to Section E (Geography)," British Association Report (Dover), 1899; M. Knudsen, Hydrographical Tables (Copenhagen, 1901); Sir John Murray, " Deep-Sea Deposits and their Distribution in the Pacific Ocean," Geogr. Journal, 1902, 19, pp. 691-711, chart ; " On the Depth, Temperature of the Ocean Waters and Marine Deposits of the South Pacific Ocean," R. Geogr. Sac. of Australia, Queensland, 1907, pp. 71-134, maps and plates; J. Thoulet, Instruments et operations d'oceanographie pratique (Paris, 1908) ; Precis d'analyse des fonds sous-marins actuels et anciens (Paris, 1907) ; T. Richard, L'Oceanographie (Paris, 1907) ; List of Oceanic Depths and Serial Temperature Observations, received at the Admiralty in the year 1888 (et seq.) from H.M. Surveying Ships, Indian. Marine Survey and British Submarine Telegraph Companies (Official). Important current and temperature charts of the ocean and occasional memoirs are published for the Admiralty by the Meteorological Office in London, by the U.S. Hydrographic Office in Washington, the Deutsche Seewarte in Hamburg, and also at intervals by the French, Russian, Dutch and Scandinavian admiralties. Pilot Charts of the North Atlantic and North Pacific are issued monthly by the U.S. Hydrographic Office, and of the North Atlantic and of the Indian Ocean and Red Sea by the British Meteorological Office, giving a conspectus of the normal conditions of weather and sea. Reports of Important Expeditions.—Sir C. Wyville Thomson, The Depths of the Sea (cruises of " Porcupine " and " Lightning ") (London, 1873) ; The Atlantic (cruise of ` Challenger ") (London, 1877) ; Die Forschungsreise S.M.S. " Gazelle " in den Jahren 1874 bis 2876 (5 vols., Berlin, 1889-189o) ; Report of the Scientific Results of the Voyage of H.M.S. " Challenger " in the years 1872-1876 (5o vols., London, 188o-1895); A. Agassiz, Three Cruises of the U.S. Coast and Geodetic Survey Steamer " Blake " ... from 1877 to z88o (2 vols., Boston, Mass., 1888) ; S. Makaroff, Le Vitiaz et l'Ocean Pacifique, 2886-1889 (St Petersburg, 1894) ; S. Makaroff, The Yermak in the Ice (in Russian) (St Petersburg, 1901); The Norwegian North Atlantic Expedition (on the " Voringen "), 1876-1878 (Christiania, 1880-1900) ; Expeditions scientiflques du ' Travailleur " et du " Talisman," 188o-1883 (Paris, 1891 et seq.); Die Ergebnisse der Plankton-Expedition, 1889 (Kiel, 1892 et seq.); Resultats des campagnes scientifeques accomplies sur son yacht par Albert I" Prince Souverain de Monaco (Monaco, from 1889) ; The Danish " Ingolf " Expedition, 18o6 (Copenhagen, 1900) ; Prof. Luksch, Expeditionen S. M. Schiff " Pole " in das Mittelmeer and in das Rote Meer, Kais. Akad. Wissenschaften (Vienna, 1891-1904); Die Deutsche " Valdivia " Tief-See Expedition, 1898-1899 (Berlin, 1900) ; M. Weber, " Siboga Expedition," Petermanns Mitteilungen (1900) ; Siboga Expeditie (Leiden, 1902 et seq.) ; F. Nansen, The Norwegian North Polar Expedition, 1893-1896 (Christiania and London', 1900) ; R. S. Peake, " On the Results of a Deep-sea Sounding Expedition in the North Atlantic Ocean during the Summer, 1899 " (Extra Publ. Geogr. Soc., London) ; Bulletin des resultats acquis pendant les courses periodiques (Conseil permanent international pour l'exploration de Ia mer) (Copenhagen, 1902 seq.). Reports of many minor expeditions and.researches have appeared in the Reports of the Fishery Board for Scotland; the Marine Biological Association at Plymouth; the Kiel Commission for the Investigation of the Baltic; the Berlin Institut fur Meereskunde; the bluebooks of the Hydrographic Department; the various official reports to the British, German, Russian, Finnish, Norwegian, Swedish, Danish, Belgian and Dutch governments on the respective work of these countries in connexion with the international. co-operation in the North_ Sea; the Bulletin du musee oceanographique de Monaco (1903 seq.); the Scottish Geographical Magazine; the Geographical Journal; Petermanns Mitteilungen; Wagner's Geographisches Jahrbuch; the Proceedings and Transactions of the Royal Societies of London and Edinburgh; the Annalen der Hydrographie; and the publications of the Swedish Academy of Sciences. (O. K. ; H. R. M.)
End of Article: TERRIGENOUS

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